Potassium titanyl phosphate (KTiOPO.sub.4 ; KTP) is a non-linear optical material that belongs to the family of compounds that have the formula unit MTiOXO.sub.4, where M can be K, Rb, Tl, NH.sub.4 or Cs (partial) and X can be P or As. In single crystal form, KTP has been shown to be useful in a broad range of applications for modulation and frequency conversion of laser radiation. KTP has been shown to be particularly useful as a "doubler" for laser systems to obtain a different frequency radiation from a laser having a fundamental frequency, such as Nd lasers emitting 1 micron radiation. This is described in U.S. Pat. No. 3,949,323 to Bierlein, et al. entitled "CRYSTALS OF (K, Rb, Tl, NH.sub.4)TiO(P, As)O.sub.4 AND THEIR USE IN ELECTROOPTIC DEVICES".
KTP is valuable in such laser systems because the material has a unique combination of properties. The properties include a large temperature bandwidth, relatively good thermal properties, large nonlinear optical coefficients which are phase matchable, wide acceptance angles and a relatively good resistance to bulk optical damage. KTP also has a high optical transmission (or a low optical absorption) for radiation having wavelengths in the long wavelength visible and the near-infrared (IR) and the mid-IR spectral regions. For such low frequency radiation, KTP is essentially transparent. It is this property (in combination with certain of the other properties) that permits KTP to have superior operating characteristics as a doubler and a modulator for lasers and other optical devices. The basic properties and the applications of KTP are described in the Journal of the Optical Society of America B, Vol. 6, No. 4, "Potassium Titanyl Phosphate: Properties and New Applications" by John D. Bierlein and Herman Vanherzeele, pages 622-633, dated April 1989.
However, it has been found that the optical transmission of KTP for radiation having wavelengths in the visible and the near-ultraviolet (UV) spectral regions is substantially reduced. Further, such reduction occurs at radiation wavelengths significantly distant from the start of the UV cutoff. This characteristic limits the use of KTP as an optical element in lasers and other optical applications. For example, lasers using a KTP crystal for visible and near-UV radiation applications experience a reduced optical frequency conversion efficiency. Further, in such applications, lasers using a KTP crystal undergo excess crystal heating and have a reduced threshold with respect to laser-induced crystal damage.
Consequently, there is a need to improve the optical transmissivity of KTP overall and for radiation having wavelengths in the visible and the near-UV spectral regions. There is also a need to increase the overall optical frequency conversion efficiency of lasers and other optical devices using a KTP crystal as an optical element for visible and near-UV radiation applications. There is also a need to reduce the crystal heating and to increase the threshold of laser-induced crystal damage of lasers and other optical devices using a KTP crystal as an optical element.